The present invention relates generally to a system and method for determining a subject spatial location, and more specifically to a system and method for using high-precision counter differentials to determine such locations with high resolution.
Monitoring the location of a person or subject has been limited to “electronic monitoring” using a radio frequency transmitter attached to a subject. The transmitter communicates to a field monitoring device (FMD), which in turn communicates with a central data base system. FMD systems can report when the subject is within a hundred feet of the FMD, as described in U.S. Pat. No. 4,918,432. Once the subject leaves the immediate area of the FMD, the location of the subject is unknown.
Other techniques for locating a subject at a predetermined area utilizes the combination of caller identification and voice recognition, such as that disclosed in U.S. Pat. No. 5,170,426; using Global Positioning System (GPS) satellites; or by polling the subject's portable tracking apparatus by placing a cellular phone call to the apparatuses as disclosed in U.S. Pat. No. 5,461,390.
These location techniques are limited in the ability to locate the subject as they move about, are costly to procure and operate due to frequent wireless radio frequency (RF) communications. Moreover, such systems generally provide poor resolution. The resolution of commercially available non-military GPS-based systems is thirty feet.
Recently, a low power, all body worn tracking apparatus was described in pending patent application entitled “BODY WORN ACTIVE AND PASSIVE TRACKING DEVICE”, Ser. No. 09/181,244, filed Oct. 28, 1998. This device, while significantly reducing size, weight, and power costs associated with tracking a subject, still incurs size, weight and power associated with wireless radio frequency (RF) communications in a wide area wireless infrastructure.
Clearly, size, weight and power are critical requirements for any body worn locating device. Recent technology described above affords the ability to combine the elements into a wristwatch form factor to provide an all body worn location recording device.
There exists a need to implement a location recording apparatus for the purpose of recording a subject's movements, such as an ambulatory patient, emergency personnel in a building, and the like. Active and passive tracking systems described in U.S. Pat. No. 5,731,757 and the functionality described in U.S. patent application Ser. No. 09/082,313 into a single body worn tamper resistant active tracking apparatus are not required to simply record the locations of a subject.